1. Field of the Invention
The present invention relates to a circuit for stabilizing a high tension voltage of a cathode ray tube (CRT) and a method thereof, and more particularly, to a circuit for stabilizing a high tension voltage of a CRT which has a function of compensating for a feed-forward extra high tension (EHT) voltage, and a method thereof. The present invention is based on Korean Patent Application Nos. 2001-56670 and 2002-28023, which are incorporated herein by reference.
2. Description of the Related Art
In general, a CRT applies a high tension voltage, namely a few tens of kV of tension to an anode, and accelerates electrons so that an electron beam emitted from a cathode can crash into a screen coated with phosphor at a high speed. Accordingly, an electron beam current, namely a cathode current runs from the anode to the cathode. While a screen image is being scanned, the direct current electron beam is modulated by an RGB image signal. Here, the modulated electron beam generates variations in the strength and weakness of the electron beam crashing into the phosphor of the screen, thereby displaying a color screen.
Since the image signal is modulated and displayed on the high tension current in the CRT, the variations of the high tension voltage influences the brightness and contrast of the screen. Therefore, the high tension voltage must be stabilized to improve quality of image.
The high tension voltage of the CRT is obtained by rectifying a flyback transformer (FBT) pulse. However, the high tension voltage power has a poor voltage variation ratio, and the high tension voltage is changed in every variation of the images. Accordingly, variations of an amplitude of a raster and deviations of a convergence and focus may take place, or the power voltage of the respective units obtained by using the FBT pulse may be changed, which results in unstable operations of the CRT. Therefore, a high tension voltage generating circuit needs measures for stabilizing a high tension voltage.
In the conventional art, in order to stabilize the high tension voltage, the high tension voltage compensation is performed by using the feedback voltage inputted from an FBT to an auto brightness limit (ABL). FIG. 1 is a block diagram illustrating a conventional circuit for stabilizing a high tension voltage.
Referring to FIG. 1, the conventional circuit 150 for stabilizing the high tension voltage includes a feedback unit 152, a comparing unit 154, a voltage output unit 156 and a driving unit 158.
The feedback unit 152 receives a voltage of an FBT 120 through an ABL 130 according to variations of the high tension voltage of a CRT 110. The comparing unit 154 extracts a variation quantity of the voltage from the feedback unit 152. The voltage output unit 156 outputs a voltage having the compensated high tension voltage on the basis of the voltage variation quantity from the comparing unit 154. The driving unit 158 outputs the compensated high tension voltage from the voltage output unit 156 to a vertical driving unit 170 and a horizontal driving unit 180. Here, the compensated high tension voltage is an E/W signal inputted to the horizontal driving unit 180.
However, with the conventional circuit for stabilizing the high tension voltage, the CRT has an image distortion around a boundary between a bright region and a dark region of the screen by the influence of the unstable compensation for high tension voltage. FIGS. 2A and 2B show the image distortion occurring in the conventional CRT. FIGS. 2A and 2B show image distortion areas 220a, 220b occurring around the boundary between the bright areas 200a, 200b and the dark areas 210a, 210b. Here, position and size of the white varies depending on the factors like image display device type, location of the image display device and size of the white bar, etc. Accordingly, complete compensation is possible when the compensation quantity of high tension voltage is varied appropriately. The conventional circuit for stabilizing the high tension voltage, however, has a problem in that it performs a fixed compensation irrespective of such factors.
On the other hand, a YUV signal is transformed into an RGB signal in a matrix circuit 142, and inputted to an image amplifying unit 160. The image amplifying unit 160 amplifies the inputted RGB signal, and outputs it to the CRT. The RGB signal amplified in the image amplifying unit 160 is fed back to a video matrix chip 140, and used to adjust white balance. In addition, a high tension voltage compensating capacitor 190 serves to reduce variations of the high tension voltage inputted from the FBT to the CRT.
However, the conventional circuit has a disadvantage in that an input signal may be delayed by the ABL circuit, because the variations of the high tension voltage are fed back through the ABL circuit. Such a signal delay is one of the main factors having a negative effect on precise phase magnitude compensation. Moreover, alternating current (AC) elements are almost removed through a low pass filter of the ABL circuit, and thus partial compensation is hard to perform.
On the other hand, when the high tension voltage compensating capacitor is used to solve the foregoing problem, it reduces the variations of the high tension voltage, but increases expenses. Further, an active correction of image is not possible as the user is not provided with the means to enable him/her to directly correct the image distortion occurring due to inaccurate compensation of high tension voltage.